This randomized, double-blind, placebo-controlled study will evaluate the safety, tolerability, pharmacokinetics and pharmacodynamics of RO5095932 in adult patients with type 2 diabetes mellitus. Patients that are on stable metformin therapy are eligible to enter the study. There will be a 5-weeks exenatide run in period and a 5-weeks period when co-administered with the study drug. RO5095932 will be administered subcutaneously as a single dose or as escalating doses. Patients will be randomized to receive either active drug or placebo in weeks 1 to 4. Active drug or placebo will be administered once weekly for 4 weeks. Patients who received active drug in weeks 1 to 4 will receive placebo in week 5. Patients who received a single dose of placebo in weeks 1 to 4, will receive a single dose of active drug in week 5. The anticipated time of study treatment is <6 months. The target sample size is <100 patients.